1. Field of the Invention
The present invention generally relates to continuous-tone facsimile devices, and more particularly to controlling the facsimile transmission rate of a continuous-tone image based on the data rate.
2. Description of the Related Art
Facsimile mechanisms for black and white facsimile transmission have been available for a substantial period of time. Recently, the International Telecommunication Union (ITU) has defined a continuous-tone data representation standard in order to make it possible to interchange continuous-tone image data (e.g., color or gray scale data) over a facsimile communication service such as Group 4 or Group 3 facsimile devices.
The standard for image encoding of continuous-tone data is based upon ITU-T Recommendation T.81(JPEG), DIGITAL COMPRESSION AND CODING OF CONTINUOUS-TONE STILL IMAGES, and ITU-T Recommendation T.42, CONTINUOUS-TONE COLOUR REPRESENTATION METHOD FOR FACSIMILE, which specifies color space representation. The techniques for image transfer applied to Group 3 facsimile are a subset of Recommendation T.81 consistent with the T.81 Recommendation. The description of color components and colorimetry for color data is included in Recommendation T.42. The specification for continuous-tone color mode for Group 3 facsimile is contained in T.4 Annex G, OPTIONAL CONTINUOUS-TONE COLOUR MODE FOR GROUP 3 of ITU-T Recommendation T.4, STANDARDIZATION OF GROUP 3 FACSIMILE TERMINALS FOR DOCUMENT TRANSMISSION, the basic standard for Group 3 facsimile devices. Continuous-tone images include both color and gray-scale images.
The ITU Continuous-Tone Facsimile standard uses the baseline JPEG image compression technique prior to transmission. For this technique, the amount of compression is controlled by the values used in a quantization table that is used both in compression (on the transmission side) and decompression (on the receiver side). There has not been a correlation between the amount of compression and the data rate with which the data is being communicated. Consequently, the amount of time that it takes for a continuous-tone image to reach the facsimile receiver has been unpredictable.
Briefly, a continuous-tone facsimile mechanism provides a transmission rate control to scale an original set of quantization values, producing a scaled set of quantization values based on a data rate negotiated between a transmitter and a receiver. The scaled set of quantization values are used to produce a compressed continuous-tone image for the facsimile mechanism. Both the compressed image and the scaled set of quantization values are transmitted to the receiver. When a rate control mode of the facsimile mechanism is disabled, the scaled set of quantization values is identical to the original set of quantization values. When the rate control mode is enabled, the elements of the original set of quantization values are increased by a relatively large amount if the data rate is relatively slow and are increased by a relatively small amount if the data rate is relatively fast. The scaled set of quantization values and the data rate thus have an inverse relationship.
One advantage of this technique of transmission rate control is the ability to control the amount of data sent based on the data rate, hence fixing the communication time. In many applications, absolute quality of the transmitted image is not as important as being able to limit the amount of time required to transmit the image, even at a cost of some degradation of the quality of the image. A user of such a continuous-tone facsimile mechanism has the capability to choose between either fixing the time required to transmit the image or fixing the quality of the image and letting the time required to transmit the image vary.